This invention relates to an improved double metal cyanide catalyst which exhibits lower amounts of high molecular weight tail and to a process for preparing this improved double metal cyanide catalyst.
The preparation of polyoxyalkylene polyols comprises oxyalkyating starter compounds having active hydrogen atoms with alkylene oxides in the presence of a suitable catalyst. For many years, basic catalysts as well as DMC catalysts have been used in these oxyalkylation reactions. Base-catalyzed oxyalkylation involves addition of alkylene oxides such as propylene oxide or ethylene oxide to low molecular weight starter compound such as propylene glycol or glycerine in the presence of a basic catalyst such as potassium hydroxide (KOH) to form a polyoxyalkylene polyol.
In base-catalyzed oxyalkylation reactions, propylene oxide and certain other alkylene oxides are subject to a competing internal rearrangement which generates unsaturated alcohols. For example, when KOH is used to catalyze an oxyalkylation reaction using propylene oxide, the resulting product will contain allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the polyol increases, the isomerization reaction becomes more prevalent. As a result, 800 or higher equivalent weight poly(propylene oxide) products prepared using KOH tend to have significant quantities of monofunctional impurities. These monofunctional impurities tend to reduce the average functionality and broaden the molecular weight distribution of the polyol.
DMC catalysts, unlike basic catalysts, do not significantly promote the isomerization of propylene oxide. As a result, DMC catalysts are suitable for the preparation of polyols which have low unsaturation values and relatively high molecular weights. DMC catalysts can be used to produce polyether, polyester and polyetherester polyols which are useful in applications such as polyurethane coatings, elastomers, sealants, foams and adhesives.
DMC catalysts are known and various processes for their preparation are described in, for example, U.S. Pat. Nos. 3,278,457, 3,278,459, 3,289,505, 3,427,256, 4,477,589, 5,158,922, 5,470,813, 5,482,908, 5,545,601, 5,627,122, 5,693,584, 5,714,428, 5,900,384, 5,952,261, 5,998,672, 6,013,596, 6,291,388, 6,423,662, 6,436,867, 6,586,566, 6,696,383, 6,797,665, 6,855,658, 6,867,162, 6,884,826 and 7,223,832, as well as in WO 01/04180. DMC catalysts are typically prepared by mixing an aqueous solution of a metal salt with an aqueous solution of a metal cyanide salt in the presence of an organic complexing ligand. A precipitate forms when these two solutions are mixed together. The resulting precipitate is isolated and then washed to remove excess metals salts and alkali metal salts that may be present in the catalyst matrix as disclosed by U.S. Pat. Nos. 3,278,457 and 6,423,662, and WO 01/04180.
DMC-catalyzed oxyalkylation reactions are also known, however, to produce small amounts of high molecular weight polyol impurities with, molecular weights that are typically in excess of 100,000 Da. These high molecular weight impurities are often referred to as the “high molecular weight tail” or HMWT. In elastomers and other systems, the high molecular weight tail may interfere with hard segment phase out as well as with the alignment of hard segments responsible for strength and modulus properties. In polyurethane foam systems, polyols which have a high molecular weight propylene oxide-rich tail produce course foam cells, very tight foams, weak foams, or contribute to foam collapse.
There has been considerable effort to find ways to reduce the high molecular weight tail that forms in polyoxyalkylene polyols due to polymerization of alkylene oxides in the presence of a DMC catalyst. These include both manipulation of processing parameters such as level and placement of ethylene oxide in the chemical structure of polyether polyols and chemical modifications of the catalyst composition. Mechanical means such as high shear mixing to destroy the higher fraction of high molecular weight tail, sonication to reduce catalyst particle size, and pre-activation of the catalyst with filtration to remove the larger particles may have been considered but deemed impractical for a commercial process.
U.S. Pat. No. 5,777,177 describes a continuous addition of starter or “CAOS” process where a low molecular weight hydroxyl compound such as glycerine is added to the reactor simultaneously along with the alkylene oxide to reduce the level of high molecular weight tail formed with DMC catalyst. This technique has been combined with strategic random incorporation of ethylene oxide or other suitable comonomers during the propoxylation process to mitigate the negative effects of high molecular weight tail on preparation of flexible foam products as taught in U.S. Pat. Nos. 6,066,683 and 6,008,263.
Chemical modification of DMC catalysts is described in U.S. Pat. No. 6,051,680 where alkyl substituted reactive silane compounds are applied to the dried catalyst slurried in a suitable solvent such that the concentration of “unbonded” zinc hydroxyl groups is reduced and significant reductions in the level of high molecular weight tail are observed. Although no actual reaction rate data is provided, a slightly broadened polydispersity is noted in the examples versus the control which indicates a lower activity for the silylated DMC catalyst.
U.S. Pat. Nos. 6,696,383, 6,867,162 and 7,223,832 disclose DMC catalysts prepared from at least one metal salt, at least one metal cyanide salt, at least one organic complexing agent, at least one alkali metal salt, and optionally, at least one functionalized polymer, in which the alkali metal salt is added to the catalyst in an amount such that the catalyst includes from 0.4 to 6 wt. % of the alkali metal. These patents also disclose a process of preparing these DMC catalysts and a process for preparing polyols from these DMC catalysts. Suitable alkali metal salts include alkali metal halides such as sodium chloride, sodium bromide, lithium chloride, lithium iodide, potassium chloride, potassium bromide, potassium iodide, and mixtures thereof.
Thus far, most of the methods that involve chemical modifications of the catalyst to reduce high molecular weight tail in polyoxyalkylene polyols have the disadvantage that catalyst reactivity is reduced. In essence, this means that the reaction takes longer to complete or more catalyst is necessary to attain the same reaction rate. This includes the DMC catalysts described in U.S. Pat. Nos. 6,696,383, 6,867,162 and 7,223,832. It is necessary to increase the amount of the DMC catalysts of these references to more than about 100 ppm to of catalyst, based on total weight of final product to obtain the same reaction rate measured with about 30 ppm of a conventional high activity DMC catalyst such as that described in, for example, U.S. Pat. No. 5,482,908. Other literature such as U.S. Pat. No. 5,952,261 and U.S. Patent application 2006/0058182 disclose the addition of cyanide-free metal salts during the catalyst precipitation step to obtain highly active catalyst that are effective at 30 ppm or less but they do not disclose or claim any reductions in the amount of high molecular weight tail produced in the alkylene oxide polymerization process to make polyols.
Therefore, a need still exits for highly active DMC catalyst compositions and a process for their use where the resultant polyols contain significantly less high molecular weight tail impurities. A goal of the present invention is to provide a novel DMC catalyst that is effective in reducing high molecular weight tail in polyoxyalkylene polyols while maintaining the high reactivity of conventional DMC catalysts.